09:45 〜 10:00
[SGC33-04] Geochemical characteristics of Unzen and Pre-Unzen volcanics
キーワード：Unzen volcano, Southwest Japan Arc, Subduction recycling, Shimabara graben
The Unzen volcano is located in the Shimabara graben and the Southwest Japan rear arc region. Its unique location involving an arc and a graben makes it an ideal case for investigating the geochemical characteristics of complex volcanic systems. However, there is insufficient published geochemical data on this rear arc volcano to fully understand the origin and characteristics of the volcanic rocks.
We analysed samples of volcanic products from a wide range of compositions (basalt to dacite) that were erupted over time (pre-Unzen to Unzen) to determine the characteristics of this volcano. The geochemical characteristics of the volcanic products in this area show both arc-like and OIB-like features. The observed geochemistry in these rocks also suggests the involvement of subducted sediments and crustal components. The major and trace element abundances, as well as the Pb-Sr-Nd isotopic data, of the Pre-Unzen basalts suggest an OIB-like character which is similar to those found in nearby rear-arc volcanoes such as Fukuejima, Kita-Matsuura, and Higashi-Matsuura. Additionally, these basalts exhibit high abundances of HFSEs, with Nb and Zr reaching up to 50 ppm and 226 ppm, respectively. Some of the samples from pre-Unzen contain a high HREE and Y bearing component, similar to a sample reported from Higashi-Matsuura (Dey et al., 2023, EarthArXiv). This component exhibits a significant Ce anomaly in the Chondrite normalized REE diagram, with Ce/Ce* ranging from 0.5 to 1.0. The samples also show enriched isotopic character, with 87Sr/86Sr ranging from 0.7038 to 0.7050, 143Nd/144Nd ranging from 0.5126 to 0.5129, and 206Pb/204Pb ranging from 18.22 to 18.47, which may indicate contribution from subducted sediments. The samples show a high variation in 207Pb/204Pb, ranging from 15.4 to 15.7, compared to a relatively low and uniform 206Pb/204Pb. This trend is similar to that of nearby rear-arc volcanoes such as Kita-Matsuura (Sakuyama et al., 2014, J. Pet.) and Goto islands (Kuritani et al., 2017, Lithos), suggesting a common trend for Kyushu rear-arc basalts.
The volcanic products of the Unzen stage range in composition from andesite to dacite and exhibit a significant negative Nb anomaly (Nb/Nb* <0.5) in the primitive mantle normalized trace element pattern. The presence of enriched fluid mobile elements (such as Ba, Pb, and Sr) and depleted HREEs compared to the pre-Unzen stage suggests a higher degree of fluid-induced melting for these rocks. Contribution from crustal material is also suggested by the trace element patterns.
We analysed samples of volcanic products from a wide range of compositions (basalt to dacite) that were erupted over time (pre-Unzen to Unzen) to determine the characteristics of this volcano. The geochemical characteristics of the volcanic products in this area show both arc-like and OIB-like features. The observed geochemistry in these rocks also suggests the involvement of subducted sediments and crustal components. The major and trace element abundances, as well as the Pb-Sr-Nd isotopic data, of the Pre-Unzen basalts suggest an OIB-like character which is similar to those found in nearby rear-arc volcanoes such as Fukuejima, Kita-Matsuura, and Higashi-Matsuura. Additionally, these basalts exhibit high abundances of HFSEs, with Nb and Zr reaching up to 50 ppm and 226 ppm, respectively. Some of the samples from pre-Unzen contain a high HREE and Y bearing component, similar to a sample reported from Higashi-Matsuura (Dey et al., 2023, EarthArXiv). This component exhibits a significant Ce anomaly in the Chondrite normalized REE diagram, with Ce/Ce* ranging from 0.5 to 1.0. The samples also show enriched isotopic character, with 87Sr/86Sr ranging from 0.7038 to 0.7050, 143Nd/144Nd ranging from 0.5126 to 0.5129, and 206Pb/204Pb ranging from 18.22 to 18.47, which may indicate contribution from subducted sediments. The samples show a high variation in 207Pb/204Pb, ranging from 15.4 to 15.7, compared to a relatively low and uniform 206Pb/204Pb. This trend is similar to that of nearby rear-arc volcanoes such as Kita-Matsuura (Sakuyama et al., 2014, J. Pet.) and Goto islands (Kuritani et al., 2017, Lithos), suggesting a common trend for Kyushu rear-arc basalts.
The volcanic products of the Unzen stage range in composition from andesite to dacite and exhibit a significant negative Nb anomaly (Nb/Nb* <0.5) in the primitive mantle normalized trace element pattern. The presence of enriched fluid mobile elements (such as Ba, Pb, and Sr) and depleted HREEs compared to the pre-Unzen stage suggests a higher degree of fluid-induced melting for these rocks. Contribution from crustal material is also suggested by the trace element patterns.